1. Field of the Invention
The present invention relates to a method of production of a magnetoresistance effect device, more particularly, relates to a method of production of a magnetoresistance effect device suitable for preventing a drop in a magnetoresistance ratio.
2. Description of the Related Art
Magnetic random access memories (MRAMs) are integrated circuit magnetic memories that is now paid attention as memories having integration densities on a par with dynamic random access memories (DRAMs), featuring high speed performance on a par with static random access memories (SRAMs), and enabling unlimited rewriting. The magnetic random access memories and magnetic heads are mainly comprised of layers of nonmagnetic or magnetic thin films of several nm (nanometer) thickness giving a tunneling magnetoresistive (TMR) effect. This configuration with the TMR effect is hereinafter referred to as “TMR devices”.
Further, each of the plurality of magnetic films forming a TMR device is formed by sputtering. The insulation layer is formed utilizing an oxidation reaction of a metal.
The basic structure of a TMR device is shown in FIG. 5. A TMR device 101 is basically comprised, as explained above, of an insulation layer 102 sandwiched at its two sides by ferromagnetic layers 103 and 104. Arrows 103a and 104a show the directions of magnetization of the ferromagnetic layers 103 and 104, respectively. FIGS. 6A and 6B are used for explaining the state of resistance in the TMR device 101 when applying a voltage V to the TMR device 101 by a DC power source 105. The TMR device 101 has the feature of changing in resistance in accordance with the states of magnetization of the ferromagnetic layers 103 and 104 due to the DC voltage V applied from the DC power source 105. Further, as shown in FIG. 6A, when the directions of magnetization of the ferromagnetic layers 103 and 104 are the same, the resistance value of the TMR device 101 becomes minimum, while as shown in FIG. 6B, when the directions of magnetization of the ferromagnetic layers 103 and 104 are opposite, the resistance value of the TMR device becomes maximum. The minimum resistance of the TMR device is indicated by “Rmin”, while the maximum resistance of the TMR device 101 is indicated by “Rmax”. Here, in general, there are a “current-in-plane” (CIP) structure sending a sense current in parallel to the plane of the device film and a “current perpendicular to plane” (CPP) type sending a sense current in a direction perpendicular to the plane of the device film. FIG. 5 and FIGS. 6A and 6B show an example of a CPP type magnetoresistance effect device.
A “MR ratio (magnetoresistance ratio)” is defined for the above TMR device 101 as follows:MR ratio=(Rmax−Rmin)/Rmin  (1)
Next, the conventional method of production and problems of a TMR device having the above multilayer structure and resistance characteristics will be explained from the viewpoint of the deterioration of the MR ratio.
A magnetoresistance effect device like a TMR device built into an MRAM or magnetic head is microprocessed in the process of production. For example, when etching the magnetic layers forming the TMR device, an etching gas of a mixed gas of carbon monoxide and a nitrogen compound (for example, NH3: ammonia) (CO+NH3) or an alcohol-based etching gas including hydroxy groups (CH3OH) etc. is used. At this time, if using a resist mask made of an organic material for the etching, no selectivity can be obtained and microprocessing is not possible, so the practice has been to use tantalum (Ta), titanium (Ti), etc. giving selectivity with respect to the magnetic layers as a hard mask and etch by reactive ion etching (RIE) etc. In particular, Ta is originally used as a thin film material forming part of a TMR device and has the advantage that it can be deposited by the sputtering method in the same step as another magnetic material (see Japanese Patent No. 3131595, Japanese Patent Publication (A) No. 2002-38285, and Japanese Patent Publication (A) No. 2001-274144).
However, if using Ta, Ti, etc. as a hard mask for etching by the above-mentioned gases, the oxygen contained in the gases reacts with the surface of the hard mask to form oxide films at the surface-most layer of the hard mask. This state will be explained with reference to FIGS. 7A to 7C. FIGS. 7A to 7C show the conventional process of dry etching of the TMR device 101 provided with a Ta layer 111 as the layer forming the hard mask. In the TMR device 101, in particular reference numeral 112 shows a substrate and 113 a bottom electrode.
In FIGS. 7A to 7C, the resist 114 is used to etch the Ta layer 111 to form a hard mask of the Ta layer 111. In the state of FIG. 7B, as a result, a hard mask 111a made of the Ta layer 111 is formed. In FIG. 7B, the hard mask 111a of the Ta layer 111 is used for etching the layers forming the TMR device. At this time, the above-mentioned gases are used. In FIG. 7C, an example of the state where the TMR device 101 is etched using the hard mask 111a of the Ta layer 111 is shown. In this example, the free layer 115 and barrier layer 116 are etched and the hard mask 111a of the Ta layer 111 is left at a thickness of tens of Å on the free layer 115.
In the conventional dry etching method explained above, finally the hard mask 111a of the Ta layer 111 is left as a surface layer at the topmost layer of the TMR device 101. After this, the TMR device 101 finished being microprocessed in the vacuum dry etching apparatus is taken out of the dry etching apparatus, that is, is exposed to the air. Therefore, the TMR device 101 is placed in an environment in contact with oxygen contained in the air. Accordingly, by not removing all of the Ta layer used as the hard mask in the etching, but leaving some of it, this is given the role of a protective layer protecting the magnetic layers from oxidation etc. As a result, the surface-most layer of the remaining hard mask 111a of the Ta layer 111 inevitably reacts with the oxygen in the air in the step of transfer from the vacuum microprocessing to the air, even if not using an etching gas containing oxygen atoms as explained above, and forms an oxide film (or oxide layer) 117 at the surface-most layer where the hard mask contacts the oxygen.
However, if the oxide film 117 is formed at the surface-most layer of the hard mask 111a of the Ta layer 111, the oxide film 117 becomes an insulation layer. If the insulation layer is formed at the surface-most layer of the TMR device 101, a parasitic resistance ends up being formed, so the above-mentioned MR ratio falls. This drop in the MR ratio is more striking with a TMR device of the CPP type, that is, the type shown in FIGS. 7A to 7C, sending a sense current in a direction perpendicular to the plane of the device film, compared with the CIP type sending a sense current in a direction parallel to the plane of the device film. Therefore, it is necessary to remove the oxide film 117 at the surface-most layer of the hard layer 111a of the Ta layer 111 to prevent a drop in the MR ratio, but as explained above, it is necessary to protect the free layer 115 from oxygen in the air when transferring the TMR device finished being microprocessed by etching to the outside of the dry etching apparatus. This problem similarly occurs when using Ti as a hard mask instead of Ta.
The TMR device after microprocessing is exposed once to the air, then transferred to the next step of production for forming electrodes etc.
Further, as related art of the present invention, there are the thin film device and method of production of the same disclosed in Japanese Patent Publication (A) No. 2001-28442. The thin film device and method of production of this publication relate to a magnetic head and a method of production of a magnetic head and mainly has as its object formation of lead electrodes by highly selective dry etching without damaging (etching) the GMR.